Hey there,
Many moons have passed since you posted, but I'll respond anyway.
I'm still not absolutely sure what the issue is, but speaking to some of the veterans over here in Finland, they suspected it's the lifter assembly. (Specifically, wear on the small protruding bit on the disc pictured at the bottom.) They even went as far to guess I could fix it by simply adjusting the clutch lever, but I got myself the parts to replace the lifter assembly.
As you can tell, I haven't yet got around to pulling the clutch apart, but I'm hoping to do that before driving season arrives. Luckily (in a sense), that's still a fair bit of time away...